1. Field of the Invention
The present invention relates to a power output apparatus, and control method therefor.
2. Description of the Related Art
To date, a number of techniques are applied for reducing hydrocarbon emissions, which are harmful substances within the exhaust gas of an engine. For example, a catalyst inside the exhaust system of an engine is effective for hydrocarbon discharge reduction. The catalyst is activated by being warmed by the exhaust gas from the engine, whereby it exhibits the ability to reduce hydrocarbon emissions.
There are three main methods to reduce hydrocarbon emissions prior to activation of the catalyst. One of the methods is to delay the engine ignition timing in one cycle to the retard side by a predetermined value or more. The technique of delay the engine ignition timing in one cycle to the retard side by a predetermined value or more shall hereinafter be referred to simply as retarding the ignition timing.
The second method is to set a valve overlap period, during which both the intake and exhaust valves are open, to a predetermined value or higher. Where valve overlap is set to a predetermined value or higher, combustion gases are blown back to the intake port, and thus hydrocarbons are also blown back to the intake port as well. Hydrocarbon discharge from the exhaust port is reduced by an extent commensurate with the portion blown back to the intake port, thereby making it possible to reduce hydrocarbon emissions.
The third method involves shifting the air/fuel ratio to lean. Carbon is contained in the gasoline normally used as engine fuel. If the air/fuel ratio, i.e. the ratio of air to fuel supplied to the engine, is lean, the amount of fuel will be a relatively small, thereby reducing the quantity of carbon, which is of course a constituent of hydrocarbons, making it possible to reduce hydrocarbon emissions.
In JP6-141405A, it is proposed to control the intake airflow to the engine to a low flow rate when the catalyst is in a non-heated state, in order to reduce the amount of exhaust and hydrocarbon emissions. In JP8-28417A, it is proposed to retard the ignition timing in order to reduce hydrocarbon emissions.
If, however, the quantity of intake air to the engine is maintained at a low flow rate until the catalyst is activated, the amount of exhaust available for activating the catalyst will also be less, and consequently the period needed to warm-up the catalyst will be longer, resulting in an increase in the total amount of hydrocarbons discharged during that period.
For the reasons indicated below, these three methods for reducing the hydrocarbon discharge before activating the catalyst are not readily applicable just after starting the engine. The first method for reducing hydrocarbon emissions involves retarding the ignition timing. Because combustion can easily become unstable directly after starting the engine, however, there is a danger of misfire occurring due to the retard of the ignition timing.
The second method for reducing hydrocarbon emissions involves setting valve overlap to a predetermined value or higher. When starting the engine, however, valve overlap must be set to a small amount in order to ensure stable idling. Furthermore, because the engine rotation will not increase the oil pressure enough to change the valve timing when the engine starts, the valve timing may not be changed. According, it is difficult to set the amount of valve overlap to a predetermined value or higher just after starting the engine, making it difficult to implement the second method.
The third method to for reducing hydrocarbon emissions involves a lean air/fuel ratio. However, because combustion is unstable directly after a cold start of the engine, if a large amount of fuel is not injected, there is a danger of misfire, and conversely there is actually an increase in hydrocarbon emissions. In other words, it is preferable to have a rich air/fuel ratio. In addition, directly after cold starting the engine, the level of friction is high and there is a great deal of variation in the condition of the intake ports of each cylinder. Because of this, it is preferable to have a rich air/fuel ratio in order to stabilize the engine rotation and enable stable combustion in all the cylinders. Thus, the third method is also difficult to implement.